Martin Wilson Lecture 4 slide1 JUAS February 2015 Lecture 4: Quenching and Protection Plan the quench process decay times and temperature rise propagation of the resistive zone computing resistance growth and decay times mini tutorial quench protection schemes LHC quench protection the most likely cause of death for a superconducting magnet

Martin Wilson Lecture 4 slide2 JUAS February 2015 Magnetic stored energy Magnetic energy density at 5T E = 10 7 Joule.m -3 at 10T E = 4x10 7 Joule.m -3 LHC dipole magnet (twin apertures) E = ½ LI 2 L = 0.12H I = 11.5kA E = 7.8 x 10 6 Joules the magnet weighs 26 tonnes so the magnetic stored energy is equivalent to the kinetic energy of:- 26 tonnes travelling at 88km/hr coils weigh 830 kg equivalent to the kinetic energy of:- 830kg travelling at 495km/hr

Martin Wilson Lecture 4 slide3 JUAS February 2015 The quench process it grows by thermal conduction stored energy ½LI 2 of the magnet is dissipated as heat greatest integrated heat dissipation is at point where the quench starts at a point - this is the problem! resistive region starts somewhere in the winding maximum temperature may be calculated from the current decay time via the U  function (adiabatic approximation) internal voltages much greater than terminal voltage ( = V cs current supply)

Martin Wilson Lecture 4 slide4 JUAS February 2015 The temperature rise function U (  ) GSI 001 dipole winding is 50% copper, 22% NbTi, 16% Kapton and 3% stainless steel Adiabatic approximation NB always use overall current density J(T) = overall current density, T = time,  = overall resistivity,  = density  = temperature, C  ) = specific heat, T Q = quench decay time. fuse blowing calculation household fuse blows at 15A, area = 0.15mm 2 J = 100Amm -2 NbTi in 5T J c = 2500Amm -2

Martin Wilson Lecture 4 slide5 JUAS February 2015 Measured current decay after a quench Dipole GSI001 measured at Brookhaven National Laboratory

Martin Wilson Lecture 4 slide6 JUAS February 2015 Calculating temperature rise from the current decay curve  J 2 dt (measured) U(  ) (calculated)

Martin Wilson Lecture 4 slide7 JUAS February 2015 Calculated temperature calculate the U(  ) function from known materials properties measure the current decay profile calculate the maximum temperature rise at the point where quench starts we now know if the temperature rise is acceptable - but only after it has happened! need to calculate current decay curve before quenching

Martin Wilson Lecture 4 slide8 JUAS February 2015 Growth of the resistive zone the quench starts at a point and then grows in three dimensions via the combined effects of Joule heating and thermal conduction *

Martin Wilson Lecture 4 slide9 JUAS February 2015 Quench propagation velocity 1 resistive zone starts at a point and spreads outwards the force driving it forward is the heat generation in the resistive zone, together with heat conduction along the wire write the heat conduction equations with resistive power generation J 2  per unit volume in left hand region and  = 0 in right hand region. where: k = thermal conductivity, A = area occupied by a single turn,  = density, C = specific heat, h = heat transfer coefficient, P = cooled perimeter,    resistivity,  o = base temperature Note: all parameters are averaged over A the cross section occupied by one turn assume x t moves to the right at velocity v and take a new coordinate   = x-x t = x-vt xtxt tt v oo resistive superconducting temperature distance

Martin Wilson Lecture 4 slide10 JUAS February 2015 Quench propagation velocity 2 when h = 0, the solution for  which gives a continuous join between left and right sides at  t gives the adiabatic propagation velocity recap Wiedemann Franz Law .k(  ) = L o  what to say about  t ? in a single superconductor it is just  c but in a practical filamentary composite wire the current transfers progressively to the copper JcJc oo ss cc J op current sharing temperature  s =   + margin zero current in copper below  s all current in copper above  c take a mean transition temperature  t = (  s +  c ) / 2 oo ss tt cc  eff  Cu

Martin Wilson Lecture 4 slide11 JUAS February 2015 Quench propagation velocity 3 the resistive zone also propagates sideways through the inter-turn insulation (much more slowly) calculation is similar and the velocity ratio  is: Typical values v ad = ms -1  v vv vv so the resistive zone advances in the form of an ellipsoid, with its long dimension along the wire heat diffuses slowly into the insulation, so its heat capacity should be excluded from the averaged heat capacity when calculating longitudinal velocity - but not transverse velocity if the winding is porous to liquid helium (usual in accelerator magnets) need to include a time dependent heat transfer term can approximate all the above, but for a really good answer must solve (numerically) the three dimensional heat diffusion equation or, even better, measure it! Some corrections for a better approximation because C varies so strongly with temperature, it is better to calculate an averaged C by numerical integration

Martin Wilson Lecture 4 slide12 JUAS February 2015 Resistance growth and current decay - numerical start resistive zone 1 temperature of each zone grows by  1  J     t /  C(  1    J 2  (  2 )  t /  C(    n  J 2  (  1 )  t /  C(  n  in time  t add zone n: v.  t longitudinal and .v.  t transverse vtvt  vdt when I  0 stop resistivity of each zone is    2  n  resistance r 1 =    f g1 (geom factor) r 2 =  2 ) * f g2 r n =  n ) * f gn calculate total resistance R =  r 1 + r 2 + r n.. and hence current decay  I = (I R /L)  t calculate resistance and hence current decay  I = R / L.  t resistivity of zone 1 is  1  in time  t zone 1 grows v.dt longitudinally and .v.dt transversely vtvt  vdt * temperature of zone grows by  1  J 2  1 )  /  C(  1 

Martin Wilson Lecture 4 slide13 JUAS February 2015 Quench starts in the pole region * * * * * * the geometry factor f g depends on where the quench starts in relation to the coil boundaries

Martin Wilson Lecture 4 slide14 JUAS February 2015 Quench starts in the mid plane * * *

Martin Wilson Lecture 4 slide15 JUAS February 2015 Computer simulation of quench (dipole GSI001) pole block 2nd block mid block

Martin Wilson Lecture 4 slide16 JUAS February 2015 OPERA: a more accurate approach solve the non-linear heat diffusion & power dissipation equations for the whole magnet

Martin Wilson Lecture 4 slide17 JUAS February 2015 Compare with measurement can include ac losses flux flow resistance cooling contact between coil sections but it does need a lot of computing Coupled transient thermal and electromagnetic finite element simulation of Quench in superconducting magnets C Aird et al Proc ICAP 2006 available at

Martin Wilson Lecture 4 slide18 JUAS February 2015 It is often useful to talk about a magnet quench decay time, defined by: i) For the example of magnet GSI001, given in Lecture 4, T d = sec Use the U(  m ) plot below to calculate the maximum temperature. ii) This was a short prototype magnet. Supposing we make a full length magnet and compute T d = 0.23 sec. - should we be worried? iii) If we install quench back heaters which reduce the decay time to 0.1 sec, what will the maximum temperature rise be? Data Magnet current I o = 7886 Amps Unit cell area of one cable A u = 13.6 mm 2 Mini Tutorial: U  function

Martin Wilson Lecture 4 slide19 JUAS February 2015 U(  m ) function for dipole GSI001

Martin Wilson Lecture 4 slide20 JUAS February 2015 Methods of quench protection: 1) external dump resistor detect the quench electronically open an external circuit breaker force the current to decay with a time constant calculate  max from where Note: circuit breaker must be able to open at full current against a voltage V = I.R p (expensive)

Martin Wilson Lecture 4 slide21 JUAS February 2015 Methods of quench protection: 2) quench back heater power a heater in good thermal contact with the winding Note: usually pulse the heater by a capacitor, the high voltages involved raise a conflict between:- -good themal contact -good electrical insulation method most commonly used in accelerator magnets detect the quench electronically this quenches other regions of the magnet, effectively forcing the normal zone to grow more rapidly  higher resistance  shorter decay time  lower temperature rise at the hot spot  spreads inductive energy over most of winding

Martin Wilson Lecture 4 slide22 JUAS February 2015 Methods of quench protection: 3) quench detection (a) not much happens in the early stages - small dI / dt  small V but important to act soon if we are to reduce T Q significantly so must detect small voltage superconducting magnets have large inductance  large voltages during charging detector must reject V = L dI / dt and pick up V = IR detector must also withstand high voltage - as must the insulation internal voltage after quench I V t

Martin Wilson Lecture 4 slide23 JUAS February 2015 Methods of quench protection: 3) quench detection (b) ii) Balanced potentiometer adjust for balance when not quenched unbalance of resistive zone seen as voltage across detector D if you worry about symmetrical quenches connect a second detector at a different point detector subtracts voltages to give adjust detector to effectively make L = M M can be a toroid linking the current supply bus, but must be linear - no iron! i) Mutual inductance D

Martin Wilson Lecture 4 slide24 JUAS February 2015 Methods of quench protection: 4) Subdivision resistor chain across magnet - cold in cryostat current from rest of magnet can by-pass the resistive section effective inductance of the quenched section is reduced  reduced decay time  reduced temperature rise often use cold diodes to avoid shunting magnet when charging it diodes only conduct (forwards) when voltage rises to quench levels connect diodes 'back to back' so they can conduct (above threshold) in either direction current in rest of magnet increased by mutual inductance  quench initiation in other regions

Martin Wilson Lecture 4 slide25 JUAS February 2015 Inter-connections can also quench photo CERN any part of the inductive circuit is at risk coils are usually connected by superconducting links joints are often clamped between copper blocks current leads can overheat link quenches but copper blocks stop the quench propagating inductive energy dumped in the link

Martin Wilson Lecture 4 slide26 JUAS February 2015 LHC dipole protection: practical implementation It's difficult! - the main challenges are: 1) Series connection of many magnets In each octant, 154 dipoles are connected in series. If one magnet quenches, the combined energy of the others will be dumped in that magnet  vaporization! Solution 1: cold diodes across the terminals of each magnet. Diodes normally block  magnets track accurately. If a magnet quenches, it's diodes conduct  octant current by-passes. Solution 2: open a circuit breaker onto a resistor (several tonnes) so that octant energy is dumped in ~ 100 secs. 2) High current density, high stored energy and long length Individual magnets may burn out even when quenching alone. Solution 3: Quench heaters on top and bottom halves of every magnet.

Martin Wilson Lecture 4 slide27 JUAS February 2015 LHC power supply circuit for one octant circuit breaker in normal operation, diodes block  magnets track accurately if a magnet quenches, diodes allow the octant current to by-pass circuit breaker reduces to octant current to zero with a time constant of 100 sec initial voltage across breaker = 2000V stored energy of the octant = 1.33GJ

Martin Wilson Lecture 4 slide28 JUAS February 2015 LHC quench-back heaters stainless steel foil 15mm x 25  m glued to outer surface of winding insulated by Kapton pulsed by capacitor 2 x 3.3 mF at 400 V = 500 J quench delay - at rated current = 30msec - at 60% of rated current = 50msec copper plated 'stripes' to reduce resistance

Martin Wilson Lecture 4 slide29 JUAS February 2015 Diodes to by-pass the main ring current Installing the cold diode package on the end of an LHC dipole

Inter-connections can also quench!

Martin Wilson Lecture 4 slide31 JUAS February 2015 magnets store large amounts of energy - during a quench this energy gets dumped in the winding  intense heating (J ~ fuse blowing)  possible death of magnet temperature rise and internal voltage can be calculated from the current decay time computer modelling of the quench process gives an estimate of decay time – but must decide where the quench starts if temperature rise is too much, must use a protection scheme active quench protection schemes use quench heaters or an external circuit breaker - need a quench detection circuit which rejects L dI / dt and is 100% reliable passive quench protection schemes are less effective because V grows so slowly at first - but are 100% reliable don’t forget the inter-connections and current leads Quenching: concluding remarks always do quench calculations before testing magnet